This invention relates to a sublimation type thermosensitive image transfer recording medium and to a method of producing same.
A sublimation type thermosensitive image transfer recording medium is of a type which is adapted for imagewise transferring an ink by sublimation or volatilization from a surface thereof to a recording sheet when heated imagewise by a laser beam or a thermal head with the recording sheet being superimposed thereon.
Since a sublimable dye is sublimated in the form of independent molecules in an amount corresponding to the thermal energy applied thereto, the sublimation type thermosensitive image transfer recording medium can afford half-tone images without difficulty. The sublimation image transfer recording method, however, has a drawback in that the running cost thereof is high, because an ink ribbon having sequentially aligned yellow, magenta, cyan and, if necessary, black sections is discarded after the recording, even though large portions remain unused on each color section.
To cope with this problem with the above one-time type recording medium, there has been proposed a multiple-times type ink ribbon used in the following two modes: (1) an equal mode in which an ink sheet and a receiving sheet are displaced through the same distance upon every printing operation of, for example, a thermal head, so that a first portion of the ink sheet used in one printing operation abuts on a second portion of the ink sheet used in the next printing operation and (2) an n-times mode in which the feeding distance in every printing operation of the receiving sheet is n-times (n&gt;1) that of the ink sheet (in other words, the feeding rate of the ink sheet relative to the thermal head is smaller than that of the receiving sheet) so that a first portion of the ink sheet used in one printing operation overlaps a second portion of the ink sheet used in the next printing operation (U.S. Pat. No. 4,880,768 and U.S. Pat. No. 5,049,538). The N-time mode is advantageous over the equal mode, because the length of each of the color sections in the ink ribbon can be reduced to 1/n and because the ink in each color section is generally evenly consumed.
While the control of the surface roughness as taught in U.S. Pat. No. 5,049,538 is effective in improving the retentivity of sensitivity of the recording medium in repeated use, it has been found that, with the known multiple-times recording medium, the image density is not uniform especially when the thickness of the ink layer is increased. In the multiple-times recording medium especially for use in the n-times mode, it is important that the ink layer should have a large thickness.